Mr Ngiam is Deputy Secretary (Policy) at the Ministry of Home Affairs, where he oversees policy, planning, international cooperation, research and statistics, legal and psychological services. He is concurrently the Deputy Chairman of Yellow Ribbon Singapore and sits on the Boards of the Gambling Regulatory Authority, the Inland Revenue Authority of Singapore, Singapore Gas Company Pte Ltd, Singapore Anglican Community Services, the Diocese of Singapore Development Board and the Middle East Institute (NUS).

Mr Ngiam was formerly the Chief Executive of the Energy Market Authority from 2018 to 2024, and Deputy Secretary (Industry) at the Ministry of Trade and Industry from 2012 to 2018. He has held various appointments in the Emeritus Senior Minister's Office, the Ministry of Community Development, Youth and Sports, and the Ministry of Education. 

He was awarded the Public Administration Medal (Silver) in 2016 and the Public Administration Medal (Gold) in 2024.